1. Field of the Invention
The present invention relates to a detection voltage output circuit of a charge generation type sensing device which converts the charges generated on the charge generation type sensing device due to a change in a physical quantity applied from the outside into a voltage and outputs the thus-obtained voltage.
2. Description of the Prior Art
It is generally known that some kinds of ferroelectric ceramic materials from among ceramic dielectrics, barium titanate (BaTiO.sub.3) and lead zirconate titanate (Pb.sub.1-x Zr.sub.x TiO.sub.3), for example, have both piezo-electricity and pyro-electricity. Charge generation type sensing devices (hereinunder referred to as "sensing devices") utilizing these piezo-electricity and pyro-electricity are utilized for various purposes as piezoelectric sensors and pyroelectric sensors.
For example, pyroelectric sensors are utilized as infrared sensors for detecting the infrared radiated from the human body, and an infrared sensor used as a human body detecting system such as that shown in FIG. 9 is known.
In FIG. 9, a sensing device 1 constitutes an infrared sensor for converting a minute temperature change caused by infrared into an electrical change. The detection current generated on the sensing device 1 is converted into a voltage by a current-voltage converter 2, and the voltage output from the current-voltage converter 2 is amplified by am amplifier 3. A low-pass filter 4 is provided at the subsequent stage to the amplifier 3 so as to remove the noise other than the detection signals received from the human body. The output from the low-pass filter 4 is compared with a reference voltage by a comparator 5. The reference numeral 6 denotes an output circuit composed of a digital circuit, a relay circuit and the like, and when the sensing device 1 detects the infrared radiated from the human body, the output circuit 6 supplies, for example, a visitor notice signal in accordance with the output from the comparator 5.
However, since the sensing device 1 generates charges when the spontaneous polarization of the pyroelectric changes due to a temperature change, the output current is as small as 10.sup.-11 to 10.sup.-13 A, so that in order to apply the sensing device 1 to a human body detecting system, it is necessary to convert a current into a voltage by using a resistance as high as 10.sup.11 to 10.sup.12 .OMEGA. for the current-voltage converter 2.
As such a current-voltage converter 2, a current-voltage converter 2A composed of a high resistor R.sub.H and an operational amplifier 2A.sub.1 the inverting input terminal of which is connected to the sensing device 1 so as to produce an output voltage V.sub.out from the output terminal, as shown in FIG. 10, is conventionally known. As another current-voltage converter 2, an impedance converter 2B composed of the high resistor R.sub.H, a source resistor R.sub.S and a field-effect transistor 2B.sub.1 the gate of which is connected to the sensing device 1 so as to supply an output voltage V.sub.out based on the voltage between both terminals of the source resistor R.sub.S from a source S, as shown in FIG. 11, is also known.
However, both of the above-described current-voltage converters in the prior art have the following defects due to the use of the high resistor R.sub.H of 10.sup.11 to 10.sup.12 .OMEGA..
Firstly, a delay in electrical response is generated due to the capacity components such as the capacity of the sensing device and the input capacity which an active element such as the operational amplifier 2A.sub.1 and the field-effect transistor 2B.sub.1 has and the stray capacity, and the high resistor R.sub.H. Especially, since the high resistor R.sub.H has a resistance of 10.sup.11 to 10.sup.12 .OMEGA., the time constant determined by the capacity components and the resistance is the order of 0.1 to 1 second, so that the responsiveness of the detecting system as a whole is deteriorated and the S/N of the signal obtained is lowered in the case of using the detecting system for the purposes other than the human body detection.
Secondly, in spite of the demand for a monolithic IC for the purpose of reduction in the size of a signal processing circuit and realization of a one-chip signal processing circuit, it is very difficult to produce a monolithic circuit in the prior art because the high resistor R.sub.H has a very high resistance.
Thirdly, since the high resistor R.sub.H has a very high resistance, it is difficult to reduce the deviation of resistance and there is a problem in the stability. Johnson noise produced by the high resistance is another problem.
On the other hand, a charge generation type sensing device intrinsically acts when an external force or external heat is applied thereto, so that noise is apt to be caused by a temperature change, vibration, etc. in the ambience, the noise containing various kinds of frequency components. In order to prevent malfunction caused by such noise and enhance the reliability, some known sensing devices are composed of a plurality of sensing devices, as shown in FIGS. 12 to 14.
The sensing device shown in FIG. 12 is composed of sensing devices 1A, 1A' which are connected with each other in series so as to have the opposite polarities and are connected to the impedance converter 2B. The sensing device shown in FIG. 13 is composed of sensing devices 1B, 1B' which are connected with each other in parallel so as to have the opposite polarities and are connected to the impedance converter 2B. Such structures can enhance the reliability to a certain extent, but the effect on the noise signals produced on the sensing devices separately from each other is disadvantageously small.
In order to enhance the reliability with respect to noise, the sensing device shown in FIG. 14 is composed of two pairs of sensing devices 1B, 1B' connected to each other in parallel and opposition, each pair 1B, 1B' being connected to a current-voltage converter 2 independently of another pair, and the respective output voltages are supplied through an AND circuit 7. Such structure, however, is disadvantageous not only in that it is difficult to realize a monolithic IC of the current-voltage converter 2 but also in that the number of parts is so large as to result in rise in cost.